Super-cooled water droplets can freeze on impact to a leading edge of a rotor blade or a rotorcraft when a combination of temperatures close to freezing, high speeds, and high cloud water concentrations occur. Helicopter and tilt-rotor blades or rotorcraft operating at temperatures below freezing tend to collect ice along a majority of the leading edge of the blades. As ice accumulation alters a stagnation point geometry of the blades, performance of the vehicle decreases. Unevenly distributed rotor ice adhesion can create increases in drag, flow separation, and high vibration levels. The increase in drag generated by accreted ice increases torque required to maintain lift conditions of the vehicle. Transmission or engine limits can be reached as ice thickness increases in this dangerous fluctuating environment making maintaining a given flying condition difficult for a pilot.
Ice shedding is another problem introduced by ice accretion on rotating blades. Shear stresses created by centrifugal forces at an interface between ice and the leading edge of an airfoil increases linearly with ice thickness. When shear stresses exceed an ultimate adhesive shear strength of the ice, shards of ice are released. Impact of shed ice could cause damage to the aircraft. As ice sheds unevenly, rotor mass unbalance introduces undesired vibrations and changes in the handling of the vehicle.
To avoid large amounts of ice formation on the rotor blades, industry has adopted a standard de-icing system for a limited number of helicopter models. The industry standard de-icing system uses thermal energy to melt accreted ice. Electro thermal de-ice systems are the only Federal Aviation Administration (FAA) certified and Department of Defense (DoD) accepted ice protection systems for rotorcraft.
The thermal de-icing mechanism is only run periodically in order to avoid large power consumption or excessive heating of the leading edge blade. The ice thickness can reach up to 1 cm before the thermal system is turned on. Such a system requires large amounts of energy (e.g., 3.9 W/cm2 or 25 W/in2) and contributes to an undesired increase in the overall weight and cost of the blade. The de-ice system may not allow for safe flight throughout the entire icing envelope, since the system may not keep up with severe ice accretion rates. Due to these drawbacks, many helicopters do not employ any de-icing capabilities, limiting operations of these vehicles under adverse conditions.
For rotorcraft employing de-icing systems, due to the requirement of significant amounts of power to operate such electro thermal de-ice systems, multi-rotor aircraft typically have their rotors de-iced in sequence. This reduces a peak power demand on the aircraft, but the power requirement still limits the use of the de-icing system. Furthermore, the alternating de-ice approach may not be able to keep up with severe ice accretion rates and ultimately may limit the all-weather capability of the aircraft.